So the question is why there's "a" in the first case and "the" in the second one.I personally thought that there should've been "the" in both cases.
Lack of income, mainly caused by unemployment, was a main reason.
This situation is the main reason for the low productivity and rampant corruption.
Usually there is only one "main" reason,* so usually we say "the main reason". "a main reason" seems slightly off to me. It seems to be a slightly inferior way of saying "an important reason", or similar.
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Usually there is only one "main" reason,* so usually we say "the main reason". "a main reason" seems slightly off to me. It seems to be a slightly inferior way of saying "an important reason", or similar.
* Though "one of the main reasons" seems OK.